Sounds like you need an amplifier.
However, if you put the amp in the attic it would need to be rated for the temperature.
Is the antenna in the attic or outside?
If you have a splitter, is it in the attic?

SiliconDust says not to use an antenna amplifier, as one is built into the HDHomeRun. It ...

Thanks. (That didn't show up when I did a search for temperature specifications of the device.)

I'm a newbie here, just having installed my first HDHomeRun 4K Flex. I have it installed in the attic of my home, close to my TV antenna. (I find that the signal reception is best that way compared to having the HDHomeRun elsewhere in the home at the end of a long coaxial cable.) Considering the ...